Parallel study of thermal resistance and permeability barrier stability of Enterococcus faecalis as affected by salt composition,growth temperature and pre-incubation temperature

In this study Enterococcus faecalis cells were grown to stationary phase in various conditions resulting in strong but similar variations in both cellular thermoresistance and permeability barrier stability (the temperature T_m that induced rapid dissipation of the ion concentration gradient during constant heating). Cells grown at 17–22°C were heat sensitive and barrier labile whilst cells grown at 10–13°C and 42–47°C were heat resistant and barrier stable. The thermal resistance and barrier stability in heat-sensitive cells, compared to the same parameters in heat-resistant cells, remained low after an additional culture at 43–47°C, indicating a persistent effect of culture at 17–22°C. In cells grown at 10–13°C, these parameters were as low as they were in the heat-sensitive cells, provided the growth media contained an ammonium salt (1%) which thus abolished the cold acclimation. Both parameters were reduced in cells growth at increased salinity (1–3% Na and K salts) and the reduction was more pronounced during growth at 17–22°C. Moreover, cells pre-cultured at 21°C with increased salinity (3% NaCl) displayed strong phenotypic effect during subsequent culturing which reflected in a 6°C decrease in both the optimal temperature and maximal temperature of growth. Compared to other bacterial strains, only a part of the change in membrane stability could be related to the variations in fatty acid composition. The index of unsaturation changed in accordance with the barrier stability and survival of cells. These findings support the conclusion that stability of permeability barrier as affected by the growth temperature, presence of ammonium and cultural conditions of progenitor cells was involved in thermal sensitivity and temperature-acclimation of E. faecalis.